In some television transmitter and receiver systems, chrominance information is transmitted by amplitude and phase modulating a subcarrier of determined frequency. In order to extract chrominance information transmitted in this manner, it is necessary for a receiver to produce a signal which is at the same frequency as the transmitted subcarrier, and which is also in a determined phase relationship therewith.
This result is conventionally achieved by using a local oscillator running at a frequency which is as close as possible to the transmitted subcarrier frequency, ie. at 4.43 MHz in the PAL system or at 3.58 MHz in the NTSC system, and by frequency and phase locking the local oscillator on the transmitted subcarrier, and in particular on a portion thereof known as the "color burst". In such a color TV system, the line signal comprises a low level blanking portion followed by a few cycles (about ten) of "color burst" signal, ie. an alternating signal at the chrominance subcarrier frequency, followed by the image information per se for a television scan line in the form of mixed luminance and chrominance information.
FIG. 1 is a highly schematic diagram of a conventional local oscillator circuit associated with a color burst phase locking loop. The circuit comprises a voltage controlled oscillator (VCO) 1 having a control input 2 which serves to modify the frequency and the phase of the signal at its output 3. A crystal 4 ensures a high degree of stability, and an adjustable capacitor 5 enables the frequency of the oscillator 1 to be set at manufacture. During each color burst, the signal at the output 3 is compared in a phase comparator 7 with the burst signal 6. The output from the comparator 7 is connected to a first input of a differential amplifier 9 whose other input receives a reference signal Ref. The output from the amplifier 9 is applied to the control input 2 of the VCO 1 via a switch 8 which is switched to conduct during each burst. Further, it is necessary to introduce a time constant into the loop, eg. of about fifteen lines' duration or about 1 millisecond, in order to store the control setting over one line. This time constant is symbolized in FIG. 1 by a storage capacitor 11. In practice, most of the components shown in FIG. 1 are included in an integrated circuit, although the storage capacitor 11, the adjustable capacitor 5 and the crystal 4 are not included in the integrated circuit.
The conventional type of circuit shown in FIG. 1 has several drawbacks. The principal drawback is that it requires a highly stable oscillator 1, which in practice means a crystal controlled oscillator including means for setting the free running frequency of the oscillator (ie. the adjustable capacitor 5 in the present example). This is due to the fact that the analog phase locking loop must necessarily have low gain for reasons of stability. The requirement for a free running frequency which is as close as possible to the desired frequency entails adjustment during manufacture, and this is a non-negligeable cost factor.
For example, in a conventional commercial TV circuit, the phase comparator and its associated amplifier deliver a 10 mV control signal for an error of 1.degree., and the frequency of the oscillator varies by 2 Hz per mV, ie. the loop gain is 20 Hz/.degree.. Since a tolerable phase error lies in the range of .+-.0.5.degree., this system cannot recover from frequency errors of more than 10 Hz, which explains why it is necessary to use a crystal controlled oscillator with an adjustable capacitor.
If it is desired to recover from frequency errors due to a crystal oscillator without frequency adjustment, which error may be about 300 Hz, and still retain a phase error of less than .+-.0.5.degree., the loop gain of 600 Hz/.degree. is required. It is shown below that such a loop cannot be stable. Suppose there is a phase difference y between the two inputs 3 and 6 of the comparator 7 at instant t. At the beginning of the comparison between the oscillator output signal 3 and the burst 6, the difference y will modify the frequency f of the oscillator by an amount: EQU df=600y.
While the bust lasts, ie. a period T=2.2 .mu.s, the oscillator phase is thus shifted by an amount dy, where: EQU dy=df.times.360.times.T
giving dy=600y.times.360.times.2.2.times.10.sup.-6 =0.5y approx.
At the end of the burst with which the phase is being compared, the phase difference will be approximately y-0.5y=0.5y. The output voltage from the phase comparator is stored at the end of the comparison and the variation in frequency df up to the beginning of the next burst is: EQU df=0.5y.times.600=300y,
which means that 62 .mu.s later (one scan line later) the phase will have shifted by an amount dy which is equal to: EQU dy=df.times.360.times.62.times.10.sup.-6 =300y.times.360.times.62.times.10.sup.-6 =6.7y
This phase shift is of opposite sign to the original phase difference, so the original phase difference of 0.5y has been converted into a phase difference of 6.2y, which in turn will be converted into a phase difference of 38y, etc. The system thus diverges because of its excessive loop gain.
An object of the present invention is to provide a local oscillator which is frequency and phase locked, but which avoids the above-mentioned drawbacks of conventional systems. The invention aims to enable a crystal controlled oscillator to be used without requiring the free running frequency of the oscillator to be adjustable.